Molten metal rechargeable electrochemical cell

ABSTRACT

The present invention provides rechargeable electrochemical cells comprising a molten anode, a cathode, and a non-aqueous electrolyte salt, wherein the electrolyte salt is situated between the molten anode and the cathode during the operation of the electrochemical cell, and the molten anode comprises an aluminum material; also provided are batteries comprising a plurality of such rechargeable electrochemical cells and processes for manufacturing such rechargeable electrochemical cells.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.14/324,311, filed on Jul. 7, 2014 and claims the benefit of U.S.provisional application Ser. No. 61/843,464, filed on Jul. 8, 2013, and61/988,460, filed on May 5, 2014. The entire contents of these patentapplications are incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

This invention generally relates to electrically rechargeableelectrochemical cells and batteries; specifically, electrochemical cellsand batteries that employ molten electrode materials.

BACKGROUND

There has been considerable recent interest in the subject of elevatedtemperature molten liquid metal batteries for stationary electricalstorage. This low cost energy storage technology holds promise to safelystore highly intermittent renewable energy sources such as solar andwind energy. This will help meet increasing global electricity demandwhile simultaneously reducing CO₂ greenhouse emissions. Theserechargeable (secondary) batteries consist of an anode, a cathode, and asuitable electrolyte salt that provides ionic conduction between them.

In molten metal rechargeable electrochemical cells, the cathode, theanode, and the electrolyte salt are stacked in order of decreasingliquid densities and heated to a molten state. When each of these threecomponents is molten, their differing densities cause them to naturallysettle into three, separate, horizontal, self-segregating(self-assembling), non-miscible molten levels. In this densitystratification arrangement, the lower density molten metal electrodefloats on top; the higher density molten metal electrode sinks to thebottom, and the electrolyte salt, having an intermediate density, floatsbetween the two molten metal electrodes. Since all active components aremolten, electrochemical diffusion and ion transport reactions areenhanced. And, molten metal electrodes are not susceptible to mechanicalfailure that often accompanies solid electrodes in electrochemicalcells. This generally leads to an extended cell cycle life. The factthat these electrodes are in their molten states also precludes theformation of dendrites.

Traditional molten metal rechargeable electrochemical cells aredescribed with alkaline or alkaline earth metal (e.g. magnesium) used asthe less dense anode metal and antimony, Sb, used as the more densecathode metal. These elevated temperature Mg—Sb cells use a suitable,ionically conducting electrolyte salt. When these components are heatedto a temperature of greater than or equal to their respective meltingtemperatures, stratification of each of these species occurs. The densermolten Sb cathode settles to the bottom, the molten salt electrolyte(MgCl₂—KCl—NaCl) rests in the middle, and the less dense molten Mg anodefloats on top. Adjacent molten layers are immiscible in each other.During a cell discharge reaction, at the anode, Mg metal is oxidized toform an Mg²⁺ cation that migrates through the molten salt electrolyte tothe Sb cathode where it is reduced to form neutral Mg that alloys withthe molten Sb. As Mg is incorporated into the bulk of the Sb cathodestructure, it forms an Mg—Sb alloy and the value of the subscript x inthe formed metal alloy Mg_(x)Sb gradually increases. Duringelectrochemical recharge, when electrical current flows into the cell,Mg is driven out of the Mg—Sb alloy, across the electrolyte, where itthen forms the original Mg anode.

The overall cell voltage during discharge is due to the differences inchemical potential between the metal anode when it is in its pure metalstate and when this anode is alloyed with the cathode metal.

These traditional molten electrode electrochemical cells suffer from anumber of substantial technical drawbacks. For instance, the anodematerial, the cathode material, or both comprise materials, such asmagnesium, that react violently with oxygen or air at elevatedtemperatures. Therefore, these cells require electrical heating elementsand hermetically sealed housings that restrict air or oxygen fromentering into the housing of the cell and reacting with the moltenmetals therein. In turn, this necessitates complicated and inefficientcell configurations that are required to form useful batteries.Moreover, technical problems (e.g., effective sealing and anti-corrosionmeasures) arise from the manufacture of cells that are airtight atelevated temperatures. Solutions to these technical problems add tosystem design complexity and increased cell construction costs.Traditional molten electrode cells are also limited by reducedperformance characteristics. For example, many traditional molten cellsgenerate undesirably low voltages (e.g., less than 1 V). And, in othertraditional cells, the anode material, the cathode material, or bothcomprise environmentally harmful, toxic, or otherwise expensive metals.

SUMMARY OF THE INVENTION

Electrochemical cells and batteries of the present invention solve oneor more of the problems, discussed above, inherent with traditionalelectrochemical cells.

The present invention provides electrochemical cells comprising moltenelectrode materials that are relatively inert to oxygen gas or air atelevated temperatures. In some embodiments, the electrochemical cellcomprises a housing that is not hermetically sealed from theenvironment. And, in some embodiments, the molten electrode materialscomprise abundant, inexpensive materials that are safe andenvironmentally benign.

In one aspect, the present invention provides an electricallyrechargeable electrochemical cell comprising an anode material thatcomprises an aluminum material. The advantages of using an aluminummaterial in a molten electrochemical cell include reduced cost, highcommercial availability, and relatively low chemical reactivity atelevated temperatures. The ability of anodes comprised of aluminummaterials to be used in the presence of oxygen allows for the productionof non-hermetically sealed electrochemical cells, cell stacks, andbatteries. It also allows for the use of heaters that burn a combustiblefluid to achieve or maintain cell operating temperatures.

In one embodiment, the present invention provides a rechargeableelectrochemical cell comprising a molten metal anode, a cathode (e.g.,molten or non-molten), and an electrolyte salt (e.g., a non-aqueousionically conductive electrolyte salt), wherein the electrolyte salt issituated between the molten anode and the cathode during the operationof the electrochemical cell, and the molten anode comprises an aluminummaterial (e.g., elemental aluminum, aluminum alloy, an amalgamatedaluminum, or any combinations thereof).

In another embodiment, the invention relates to a rechargeable batterycomprising a plurality of electrochemical cells that are electricallyconnected, wherein each electrochemical cell comprises a molten anode, acathode, and a molten, non-aqueous electrolyte salt, wherein the moltenanode comprises elemental aluminum, an aluminum alloy, or an amalgamatedaluminum. In one embodiment, the cells may be stacked in a bipolarconfiguration.

In another embodiment, the invention relates to a process formanufacturing an electrically rechargeable electrochemical cellcomprising heating an anode material to a molten state, wherein themolten anode material comprises aluminum; providing a cathode material;and providing an electrolyte salt situated between the anode materialand the cathode material, wherein the molten anode material has adensity that is less than the density of the cathode material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an electrochemical cell of the presentinvention with (1) a lower density molten anode, (2) an intermediatedensity electrolyte salt, and (3) a higher density cathode.

FIG. 2 is an illustration of a bipolar configuration of three cellsaccording to the present invention with (1) lower density molten anodes,(2) intermediate density electrolyte salts, (3) higher density cathodes,and (4) bipolar electrodes.

FIGS. 3-7 are plots of current (I) vs. energy (E) that were generated byCV scans of example cells of the present invention described in Example1.

FIG. 8 is a plot of current (I) vs. time (E) at cell operatingtemperatures of 815° C. and 760° C. for an example cell of the presentinvention described in Example 1 that was discharged at 0.25 V.

FIG. 9 is a plot of current (I) vs. energy (E) that was generated by theCV scan described in the Example 2.

These Figures are provided by way of example and are not intended tolimit the scope of the invention.

DETAILED DESCRIPTION

The present invention provides an electrochemical cell comprising ananode and a cathode, wherein the anode, the cathode, or both is in amolten state, and an electrolyte salt situated between the electrodematerials.

I. DEFINITIONS

As used herein, the term “battery” encompasses electrical storagedevices comprising one electrochemical cell or a plurality ofelectrochemical cells. A “secondary battery” is electricallyrechargeable, whereas a “primary battery” is not electricallyrechargeable. For secondary batteries of the present invention, abattery cathode is designated as the positive electrode during batterydischarge and the negative electrode during battery charging.Accordingly, the anode is designated as the negative electrode duringcell discharge, and as the positive electrode during cell charge.

As used herein, the terms “electrochemical cell” and “cell” are usedinterchangeably.

As used herein, the term “molten” refers to a material that has beenheated to a fluid (e.g., liquid) state.

As used herein, the term “aluminum” or “aluminum material” refers toelemental aluminum or aluminum alloys.

As used herein, the term “aluminum alloy” refers to a homogeneousmixture of elemental aluminum with one or more additional alloyingmetals. Examples of possible alloying metals include but are not limitedto iron, silicon, gallium, bismuth, cadmium, scandium, indium, lead,mercury, thallium, tin, zinc, lithium, magnesium, antimony, copper,manganese, nickel, chromium, vanadium, titanium, zirconium, or anycombination thereof.

As used herein, the term “operating temperature” refers to the averagetemperature of the materials inside the electrochemical cell when thecell is capable of discharging electrical current, or the cell is beingrecharged with electrical current.

As used herein, the term “eutectic amount” or “eutectic amounts” refersto the amount of a material or materials that, when mixed with anothermaterial, reduces the melting point of the mixture to a lowertemperature than any other composition consisting of the same materials.

As used herein, the term “alloy” or “alloyed” refers to the combinationof two or more metals for form a substantially homogenous metallicmaterial.

As used herein, the term “non-aluminum alloying material” refers to anymaterial that is substantially free of aluminum, and may be combinedwith aluminum to form an alloy.

As used herein, an “anode” is an electrode through which (positive)electric current flows into a polarized electrical device. In a batteryor galvanic cell, the anode is the negative electrode from whichelectrons flow during the discharging phase in the battery. The anode isalso the electrode that undergoes chemical oxidation during thedischarging phase. However, in secondary, or rechargeable, cells, theanode is the electrode that undergoes chemical reduction during thecell's charging phase. “Anode” and “anode material” are usedinterchangeably herein.

As used herein, a “cathode” is an electrode from which (positive)electric current flows out of a polarized electrical device. In abattery or galvanic cell, the cathode is the positive electrode intowhich electrons flow during the discharging phase in the battery. Thecathode is also the electrode that undergoes chemical reduction duringthe discharging phase. However, in secondary or rechargeable cells, thecathode is the electrode that undergoes chemical oxidation during thecell's charging phase. Cathodes are formed from electrically conductiveor semiconductive materials, e.g., metal oxides, metal sulfides, layeredcompounds, zinc-metal phosphates, zinc-metal oxides, or any combinationthereof. “Cathode” and “cathode material” are used interchangeablyherein.

As used herein, an “electrolyte salt” or “electrolyte” refers to asubstance that behaves as an electrically conductive medium. Forexample, the electrolyte facilitates the mobilization of electrons andcations (e.g., divalent or trivalent cations) in the cell.

As used herein, the term “oxide” applied to secondary batteries andsecondary battery electrodes encompasses corresponding “hydroxide”species, which are typically present, at least under some conditions.

As used herein, the term “metal oxide” includes compounds that includeat least one metal atom and at least one oxygen atom. ‘Metal oxides’include “mixed metal oxides”, wherein the metal oxide comprises at leasttwo metal atoms of different elements and at least one oxygen atom.

As used herein, the term “metal sulfide” includes compounds that includeat least one metal atom and at least one sulfur atom. ‘Metal sulfides’include “mixed metal sulfides”, wherein the metal sulfide comprises atleast two metal atoms of different elements and at least one sulfuratom.

As used herein, the term “dopant” refers to a substance added to amaterial to alter its electrical properties. A “doped” material is amaterial to which a ‘dopant’ has been applied.

As used herein, the term “manganese oxide” refers to any manganesecompound that includes one or more oxygen atoms in its coordinationsphere. Examples of manganese oxide include MnO, MnO₂, Mn₂O₃, Mn₃O₄,Mn(OH)₂, Mn(OH)₄, MnO₂(OH)₂, Mn(OH)₃, MnOOH, Mn(ONa)₂, Mn(OK)₂,Mn(OLi)₂, Mn(ORb)₂, MnOONa, MnOOK, MnOOLi, MnOORb, ZnFeMnO₂, (MnFe)₂O₃,NiMnO₄, any hydrate thereof, or any combination thereof. In otherexamples, manganese oxide has the chemical formula Mn_(x)O_(y) wherein xis greater than or equal to 1, and y is greater than or equal to 2. Someexamples of manganese oxide have the chemical formula of MnO₂, Mn₅O₈,Mn₃O₇.3H₂O, Mn₇O₁₄.3H₂O, Mn₄O₉.3H₂O, Mn₂O₄, Mn₄O₁₈.H₂O, or anycombination thereof. In other examples, the manganese oxide has apredominate crystal structure of α-MnO₂, β-MnO₂, γ-MnO₂, δ-MnO₂,layered, or any combination thereof. Note that ‘hydrates’ of manganeseinclude hydroxides of manganese. The term ‘manganese oxide’ alsoincludes any of the abovementioned species that are doped and/or coatedwith dopants and/or coatings that enhance one or more properties of themanganese.

As used herein, “vanadium oxide” refers to any vanadium compound havingat least one oxygen atom in its coordination sphere. ‘Vanadium oxide’includes oxides or hydroxide of vanadium, e.g., VO, VO₂, V₂O₃, V₂O₅,V₃O₇, V₄O₉, V₆O₁₃, V₄O₇, V₅O₉, V₆O₁₁, V₇O₁₃, V₈O₁₅, or any combinationthereof.

As used herein, the term “binary melt” refers to a substantiallyhomogeneous mixture of two materials in a molten state.

As used herein, the term “ternary melt” refers to a substantiallyhomogeneous mixture of three materials in a molten state.

As used herein, the term “hermetic seal” refers to a boundaryimpermeable to air.

As used herein, the term “bipolar configuration” or “bipolar assembly”refers to identical individual cells placed next to each other in asandwich-like stack assembly where two adjacent cells share a common,thin, electrically conductive plate electrode. In this compact,duplex-type electrode configuration, two or more series-coupledelectrochemical cells are arranged in such a way that one side of thecommon, electrically conducting electrode plate is the negative anode ofone cell and the other side of this common, electrically conductingelectrode is the positive cathode of the adjacent cell.

As used herein, the terms “first” and/or “second” do not refer to orderor denote relative positions in space or time, but these terms are usedto distinguish between two different elements or components. Forexample, a first component does not necessarily proceed a secondcomponent in time or space; however, the first component is not thesecond component and vice versa. Although it is possible for a firstcomponent to precede a second component in space or time, it is equallypossible that a second component precedes a first component in space ortime.

II. ELECTROCHEMICAL CELLS

The electrochemical cell of the present inventions comprise a moltenanode, a cathode, and a non-aqueous electrolyte salt, wherein theelectrolyte salt is situated between the molten anode and the cathodeduring the operation of the electrochemical cell, and the molten anodecomprises an aluminum material.

In some embodiments, the anode material possesses a density that islower than the density of the cathode material at the operatingtemperature of the cell. In other embodiments, the electrolyte saltpossesses a density that is greater than the density of the anodematerial at the operating temperature of the cell. And, in otherembodiments, the cathode material possesses a density that is greaterthan the density of the electrolyte salt at the operating temperature ofthe cell.

In some embodiments, the anode to forms a low density layer at theoperating temperature of the cell, the cathode forms a high densitylayer at the operating temperature of the cell, and the electrolyte saltforms an intermediate density layer that is greater than the density ofthe anode and less than the density of the cathode at the operatingtemperature of the cell.

In some embodiments, the anode material comprises a molten aluminummaterial. In other embodiments, the cathode and the electrolyte salt aremolten at the operating temperature of the cell.

In other embodiments, the anode material possesses a density of about 6g/cm³ or less. In some embodiments, the cathode material possesses adensity greater than about 6 g/cm³.

Electrolyte salts useful for the present invention mediate themobilization of electrons and/or cations when the salt is molten.Additionally, in its molten state, the density of the electrolyte saltshould be intermediate between that of the metal anode and metalcathode. This permits the electrolyte salt to self-situate between theanode and cathode.

To better illustrate the relationship between the density (when molten)of the metal anode, metal cathode, and electrolyte salt, Table 1 belowlists densities of various metals when in their molten state.

TABLE 1 Densities of Various Metals at ~700° C. Material DensitySuitability as Anode or Cathode Magnesium (Mg) ~1.5 g/cm³ Anode Aluminum(Al) ~2.5 g/cm³ Anode Barium (Ba) ~3.5 g/cm³ Anode Antimony (Sb) ~6.5g/cm³ Anode or Cathode Zinc (Zn) ~6.5 g/cm³ Anode or Cathode Tin (Sn) ~7g/cm³ Cathode Cadmium (Cd) ~8 g/cm³ Cathode Bismuth (Bi) ~10 g/cm³Cathode Lead (Pb) ~10.5 g/cm³ Cathode

The operating temperature is another consideration for theelectrochemical cell. By using operating temperatures in theelectrochemical cell that are above the melting point of the anode, theanode will exist in a molten state. In some cases, operatingtemperatures greater than 500° C., greater than 650° C., or greater than700° C. can be used. For example, the operating temperature is greaterthan 800° C.

Another aspect of the present invention provides a rechargeableelectrochemical cell comprising a molten anode comprising an aluminummaterial; a cathode comprising a bismuth material, a tungsten material,or any combination thereof; and a non-aqueous electrolyte salt, whereinthe electrolyte salt is situated between the molten anode and thecathode during the operation of the electrochemical cell, and the moltenanode comprises an aluminum material.

A. Anode

Anodes that are useful in the electrochemical cell of the presentinvention possess one or more of the following properties when molten:

-   -   1) density when molten is lower than that of the cathode;    -   2) density when molten is lower than that of the electrolyte        salt;    -   3) readily alloys with the cathode material at cell operating        temperature;    -   4) non-flammable in the presence of air or moisture at cell        operating temperature;    -   5) non-reactive with air or moisture at cell operating        temperature;    -   6) low cost and readily available in multi-ton quantities;    -   7) have little or no reaction with the cell housing or cell        sealing materials; and    -   8) should be molten at approximately the same temperature as the        electrolyte.

In some embodiments, the anode comprises an aluminum material, where thealuminum material comprises elemental aluminum or an aluminum alloy. Insome embodiments, the aluminum material has a reduced chemicalreactivity at elevated temperatures. For instance, the aluminum materialis less reactive with oxygen than elemental magnesium at elevatedtemperatures.

In another embodiment, the aluminum material forms a protective,self-limiting, aluminum oxide (e.g., Al₂O₃) layer in the presence ofair. This oxide skin protects underlying molten aluminum from furtheroxidation and gives molten aluminum added stability. Thus,electrochemical cells comprising molten aluminum materials need not behermetically sealed.

In another embodiment, aluminum material has lower vapor pressure thanmagnesium.

A comparison of vapor pressures for aluminum and magnesium is shown inTable 2 below.

TABLE 2 Comparison of Vapor Pressures for Al and Mg. Vapor Pressure Mg361 Pa (at 649° C.) Al 2.4 × 10⁻⁶ Pa (at 660° C.)

In some embodiments, the anode comprises an aluminum material and amolten salt electrolyte comprising aluminum halide and an alkali metalhalide.

In some embodiments, the aluminum material does not substantially erodeSiN at about 800° C. In some of these embodiments, the housing of theelectrochemical cell comprises SiN.

In some embodiments the aluminum material is elemental aluminum.

In some embodiments, the aluminum material comprises an aluminum alloy,and the aluminum alloy comprises aluminum alloyed with iron, silicon,gallium, bismuth, cadmium, scandium, indium, lead, mercury, thallium,tin, zinc, lithium, magnesium, antimony, copper, manganese, nickel,chromium, vanadium, titanium, zirconium, or any combination thereof.

In some embodiments, the aluminum alloy comprises aluminum alloyed witha eutectic amount of iron, silicon, gallium, bismuth, cadmium, scandium,indium, lead, mercury, thallium, tin, zinc, lithium, magnesium,antimony, copper, manganese, nickel, chromium, vanadium, titanium,zirconium, or any combination thereof.

In some embodiments, the aluminum alloy comprises aluminum alloyed withiron, silicon, gallium, lithium, or any combination thereof.

In other embodiments, the aluminum alloy comprises aluminum alloyed witha eutectic amount of iron, silicon, gallium, lithium, or any combinationthereof.

In some embodiments, the aluminum alloy may comprise from about 0.01 toabout 50 wt % of a non-aluminum alloying material. The aluminum alloymay comprise from about 0.01 to about 5 wt % of a non-aluminum alloyingmaterial, from about 5 to about 25 wt % of a non-aluminum alloyingmaterial, or from about 25 to about 50 wt % of a non-aluminum alloyingmaterial.

B. Cathode

Cathodes and cathode materials useful in electrochemical cells of thepresent invention possess one or more of the following properties:

-   -   1) density at the operating temperature of the cell is greater        than that of the anode;    -   2) density at the operating temperature of the cell is greater        than that of the electrolyte salt;    -   3) easily forms stable alloys with the anode material at the        operating temperature of the cell;    -   4) non-flammable in the presence of air or moisture at cell        operating temperature;    -   5) non-reactive with air at cell operating temperature; and    -   6) have little or no reaction with the cell container or cell        components at cell operating temperature.

When using aluminum as the anode metal, one of the factors influencingthe choice of suitable cathode metals will depend on its ability to forman alloy with Al in its molten state. The choice of suitable metals fora molten cathode in this system depends on its melting point, density,stability, and electrochemical potential. Aluminum forms alloys with anumber of metals including iron, copper, chromium, vanadium, titanium,bismuth, gallium, lead, zirconium, magnesium, manganese, silicon,antimony, tin, and zinc, any one of which or any combination of whichmay be used as a cathode in cells of the present invention.

In some embodiments, the cathode comprises a non-metallic material. Inother embodiments, the cathode is not molten at the temperatures atwhich the cell operates.

In some embodiments, the cathode material comprises an elemental metal,metal alloy, metal oxide, metal sulfide, or doped material.

In some embodiments, the cathode comprises a cathode material that has amelting temperature that is less than or equal to the meltingtemperature of the anode.

In other embodiments, the cathode comprises a cathode material that hasa melting temperature that is greater than or equal to the meltingtemperature of the anode.

In some embodiments, the cathode material is molten during the operationof the electrochemical cell.

In further embodiments, the cathode comprises iron, copper, chromium,vanadium, titanium, bismuth, gallium, lead, zirconium, magnesium,manganese, silicon, antimony, tin, zinc, or any combination thereof.

In specific embodiments, the cathode comprises elemental tin.

In other embodiments, the cathode comprises a bismuth material. In someinstances, the bismuth material comprises bismuth and tin. For example,the cathode comprises from about 40 wt % to about 60 wt % of tin andfrom about 40 wt % to about 60 wt % of bismuth. And, in some examples,the cathode comprises about 50 wt % of tin and about 50 wt % of bismuth.

In some embodiments, the cathode material is substantially solid duringthe operation of the electrochemical cell.

In other embodiments, the cathode comprises one or more metal sulfides.For example, the cathode comprises FeS₂.

In some embodiments the cathode comprises manganese oxide, vanadiumoxide, titanium aluminum tetrachloride, manganese chlorine aluminumtetrachloride, cobalt aluminum tetrachloride, or any combinationthereof.

In some specific embodiments, the cathode comprises Mn₂O₄, Ti(AlCl₄)₂,MnCl(AlCl₄), Co(AlCl₄)₂, V₂O₅, or any combination thereof.

In some specific embodiments, the cathode comprises a tungsten material.For example, the cathode comprises tungsten oxide (e.g., WO₃).

In some preferred embodiments, the cathode is capable of forming analloy with the molten anode when cathode is also in a molten state. In aspecific embodiment, the cathode is capable of forming an alloy withmolten aluminum when the cathode material is also in a molten state.

C. Electrolyte Salt

Electrolyte salts useful in the electrochemical cells of the presentinvention possess one or more of the following properties in its moltenstate:

-   -   1) its density at the cell operating temperature is lower than        that of the cathode and greater than that of the anode;    -   2) low cost;    -   3) have a melting point in the range where both the anode and        cathode melt;    -   4) non reactivity with both molten metal anode and cathode;    -   5) be an excellent ionic conductor of the anode cation;    -   5) be electrically insulating;    -   6) not become readily oxidized during cell recharge; and    -   7) not react chemically with the cell housing or sealing        materials.

In some embodiments the electrolyte salt comprises an anion selectedfrom a halide anion, a carbonate anion, a sulfate anion, a phosphateanion, an oxide anion, a sulfide anion, or any combination thereof; anda cation selected from an alkali metal cation, an alkaline earth metalcation, a rare earth metal cation, an actinide cation, a Sc cation, a Tication, a V cation, a W cation, a Mn cation, an Fe cation, a Co cation,an Al cation, an organic cation, or any combination thereof.

In other embodiments, the anion is a halide anion.

In a further specific embodiment, the anion is a chloride anion.

In a specific embodiment, the cation is an alkali metal cation.

In a further specific embodiment, the cation is a sodium cation, apotassium cation, or any combination thereof.

In another specific embodiment the cation is an aluminum cation.

In a further specific embodiment the electrolyte salt comprises analuminum halide.

In an even further specific embodiment, the aluminum halide comprisesaluminum chloride.

In some embodiments, the electrolyte salt is a salt of formula AlX₃,wherein X is a halide ion.

In some embodiments, the electrolyte salt comprises an alkali metalhalide or an alkaline earth metal halide.

In some specific embodiments, the electrolyte salt comprises NaF, KF,CaF, LiF, KCl, NaCl, CaF₂, or BaF₂, or any combination thereof. Forexample, the electrolyte salt comprises NaF, AlF₃, CaF₂, and BaF₂.

In some embodiments, the electrolyte salt comprises an alkalimetal-aluminum halide.

In some specific embodiments, the electrolyte salt comprises LiAlCl₄,LiAlBr₄, NaAlBr₄, KAlCl₄ or Na₃AlF₆, or any combination thereof.

In some embodiments, the electrolyte salt comprises aluminum oxide,vanadium oxide, tungsten oxide, calcium carbonate, potassium-calciumcarbonate, or sodium-calcium carbonate.

In some specific embodiments, the electrolyte salt comprises Al₂O₃ orV₂O₅.

In some embodiments, the electrolyte salt comprises Na₂S, FeS₂, or CoS₂.

In some embodiments, the electrolyte salt comprises a bulk materialcomprising a single cation species and a single anion species.

In a further embodiment, the electrochemical cell comprises a secondelectrolyte salt, wherein the second electrolyte salt comprises an anionand a cation, and the anion is selected from a halide anion, a carbonateanion, a sulfate anion, a phosphate anion, an oxide anion, a sulfideanion, or any combination thereof; and the cation is selected from analkali metal cation, an alkaline earth metal cation, a rare earth metalcation, an actinide cation, a Sc cation, a Ti cation, a V cation, a Wcation, a Mn cation, an Fe cation, a Co cation, an Al cation, an organiccation, or any combination thereof.

In some embodiments, the electrolyte salt comprises an aluminium halide,an alkali metal halide, an alkaline earth metal halide, an alkalimetal-aluminum halide, an aluminum oxide, a vanadium oxide, a tungstenoxide, a sodium sulfide, an iron sulfide, a cobalt sulfide, animidazolium halide, a butyl pyridinium halide, a calcium carbonate, apotassium-calcium carbonate, a sodium-calcium carbonate, or anycombination thereof.

In some embodiments, the electrolyte salt comprises a binary meltmaterial or a ternary melt material.

In some specific embodiments, the electrolyte salt comprises a binarymelt material comprising aluminum halide and an alkali metal halide.

In a specific embodiment, the binary melt material comprises AlCl₃ andKCl.

In another specific embodiment, the binary melt material comprises AlCl₃and NaCl.

In some specific embodiments the electrolyte salt comprises a ternarymelt material comprising aluminum halide, a first alkali metal halide,and a second alkali metal halide.

In one specific embodiment, the ternary melt material comprises AlCl₃,KCl, and NaCl.

In some embodiments, the electrolyte salt comprises NaF, AlF₃, CaF₂, andBaF₂, or any combination thereof formalized to insure that the densityof the molten salt is greater than that of the molten aluminum or moltenaluminum alloy.

In a more specific embodiment, the electrolyte salt comprises from about10 to about 20 wt % of NaF, from about 40 to about 60 wt % of AlF₃, fromabout 10 to about 20 wt % of CaF₂, and from about 10 to about 20 wt % ofBaF₂.

In one specific embodiment, the electrolyte salt comprises about 18 wt %of NaF, about 48 wt % of AlF₃, about 16 wt % of CaF₂, and about 18 wt %of BaF₂.

In a preferred embodiment, the anion of the electrolyte salt is notsubstantially oxidized when the electrochemical cell recharges.

D. Materials of Construction

Materials useful for the construction of the cell walls may beelectrically non-conductive, thermally insulating, and resistant toattack by molten aluminum and molten salts at operating temperatures ofthe cell. Suitable materials may include alumina and mixtures of aluminawith calcium aluminate cements.

In some embodiments, the electrochemical cell comprises a housing thatcontains the molten anode, the cathode, and the electrolyte salt,wherein a portion of the housing contacts the molten anode, and theportion of the housing that contacts the molten anode comprises anelectrically conductive material.

In another embodiment, the portion of the housing in contact with themolten anode does not substantially react with the molten anode.

In some embodiments, the portion of the housing that contacts the moltenanode comprises graphite.

In some embodiments, the housing further comprises boron carbide,aluminum boride, zirconium dioxide, or any combination thereof.

The housing may comprise an airtight chamber or a chamber that ispermeable or open to air.

III. CELL STACKS AND BATTERIES

Another aspect of the present invention provides a rechargeable batterycomprising a plurality of electrochemical cells which are electricallyconnected, wherein each electrochemical cell comprises a molten anode, acathode, and a molten, non-aqueous electrolyte salt, wherein the moltenanode comprises elemental aluminum or an aluminum alloy.

In a further embodiment, the battery comprises a heater that thermallycommunicates with the molten anode, the cathode, the molten electrolytesalt, or any combination thereof.

In a specific embodiment, the heater burns a combustible fuel or fluid(e.g., natural gas, propane, methane, syn gas, diesel fuel, gasoline,kerosene, any combination thereof, or the like) when the heater isoperating.

In some embodiments, the plurality of electrochemical cells areelectrically connected in the absence of a hermetic seal.

In some embodiments, the plurality of electrochemical cells are stackedin a bipolar configuration. Bipolar configurations have the advantagesof lower battery weight, increased effective gravimetric energy density,reduced battery assembly and materials costs, lower effectiveresistance, minimized IR losses between adjacent cells, and more uniformcurrent/potential distributions across surface area of active materialsin each cell. Cells in a bipolar configuration can handle higher powerresulting in greater overall battery energy density and can be assembledin higher voltage configurations (e.g. 24, 48, 60V).

IV. METHODS OF MANUFACTURING AN ELECTROCHEMICAL CELL

The present invention also provides a process for manufacturing arechargeable electrochemical cell comprising heating an anode materialto a molten state, wherein the molten anode material comprises analuminum material, providing a cathode material, and providing anon-aqueous electrolyte salt between the anode material and the cathodematerial, wherein the molten anode material has a density that is lessthan the density of the cathode material.

In one embodiment, the high density cathode material may be heated toits melting point to form a molten liquid. The molten liquid metal maythen be flowed into a housing to form a first layer in the housing. Anintermediate density electrolyte salt may be heated to its melting pointto form a molten liquid electrolyte salt. The electrolyte salt may thenbe flowed into the housing and form a second layer in the housing. A lowdensity anode material may be heated to its melting point to form amolten liquid and flowed into the housing to form a third layer in thehousing. The difference in the densities of the anode material, theelectrolyte salt, and the cathode material will result in the threemolten layers remaining separate with the electrolyte salt positionedbetween the molten anode and molten cathode layers.

In an alternative embodiment, the anode material, the electrolyte salt,and the cathode material are stacked in order of decreasing density,i.e., the less dense material is situated on the top of the stack,within a cell housing and heated to a temperature wherein at least theanode material becomes molten. In some of these embodiments, the anodematerial, the electrolyte salt, and the cathode material are heated to atemperature wherein each of the anode material, the electrolyte salt,and the cathode material are molten. In these specific embodiments, themolten anode material, molten electrolyte salt, and the molten cathodematerial form three distinct, non-miscible layers.

V. EXAMPLES Example 1: Bi—Sn Cathode Cells

Five cells according to the present invention were formed using a KerrAuto Electro-Melt furnace that was modified to include two holes in thecover to allow for the insertion of current collectors into the anodeand cathode. The Kerr Auto Electro-Melt furnace included a graphitecrucible placed on the inside of the furnace. Within the graphitecrucible, an alumina crucible (33 mm outer diameter, 25 mm innerdiameter, 105 mm length—McDanel Ceramics) was placed. The materials usedfor the molten metal battery were placed inside of the alumina crucible.10 g of the cathode material (50/50 weight mixture of bismuth and tingranules (1-2 mm)) was first placed in the alumina crucible. 15 g of theelectrolyte (a powder combination of 18% wt NaF, 48% wt AlF₃, 16% wtCaF₂, and 18% wt BaF₂ that has already been mixed, heated to melting inalumina crucible using a muffle furnace at 900° C., then allowed to cooland solidify) was then placed in the alumina crucible. Then, 10 g of theanode material (aluminum granules, 8-12 mm in size) was placed in thealumina crucible. The powders for the electrolyte and the anode materialare commercially available from Alfa Aesar.

The Kerr Auto Electro-Melt furnace was covered and the temperature wasraised to 815° C.

The cathode current collector, a molybdenum rod (5 mm diameter) wasinserted into an alumina tube with an inner diameter of 5 mm and anouter diameter of 8 mm such that 3 mm of the molybdenum rod was exposedfrom the end of the alumina tube. The alumina tube was used toelectrically insulate the molybdenum from the cathode and theelectrolyte. Once the furnace reached temperature, the molybdenum rodassembly was inserted in one of the two holes in the furnace cover andplaced such that the bottom of the rod was touching the bottom of thealumina crucible and the exposed molybdenum is surrounded by the moltenmetal cathode. The anode current collector, a graphite rod (5 mmdiameter) was inserted in the second of the two holes in the furnacecover and placed such that the rod is only in the molten aluminumportion of the molten metal battery. Both the molybdenum and graphitecurrent collectors are commercially available from Alfa Aesar.

The cyclic voltammetry (CV) scans, provided in FIGS. 3-7 as I vs. Eplots, were performed using a Parstat 2273 potentiostat (PrincetonApplied Research).

In the CV scan, i.e., the I vs. E plot, depicted in FIG. 3; thefollowing settings were used:

Scan Definition:

-   -   Initial Potential: 0.0000 V vs open circuit    -   Vertex Potential: −0.8000 V vs open circuit    -   Vertex Potential: 0.8000 V vs open circuit    -   Final Potential: 0.0000 V vs open circuit    -   Scan rate: 50.00000 mV/s

PreScan Definition:

-   -   Open circuit: 1.060 V (Measured)

In the CV scan, i.e., the I vs. E plot, depicted in FIG. 4; thefollowing settings were used:

Scan Definition:

-   -   Initial Potential: 1.1 000 V vs ref    -   Vertex Potential: 0.1 000 V vs ref    -   Vertex Potential: 1.9 000 V vs ref    -   Scan rate: 5 0. 00000 mV/s    -   Cycles: 3

PreScan Definition:

-   -   Open circuit: 1. 098 V (Measured)

In the CV scan, i.e., the I vs. E plot, depicted in FIG. 5; thefollowing settings were used:

Scan Definition:

-   -   Initial Potential: 0.0000 V vs open circuit    -   Vertex Potential: −1.0000 V vs open circuit    -   Vertex Potential: 1.0000 V vs open circuit    -   Final Potential: 0.0000 V vs open circuit    -   Scan rate: 50.00000 mV/s

PreScan Definition:

-   -   Open circuit: 1.469 V (Measured)    -   Equilibration Time: 15 sec

In the CV scan, i.e., the I vs. E plot, depicted in FIG. 6; thefollowing settings were used:

Scan Definition:

-   -   Initial Potential: 1.2000 V vs ref    -   Vertex Potential: 1.9000 V vs ref    -   Vertex Potential: 0.1000 V vs ref    -   Final Potential: 1.2000 V vs ref    -   Scan rate: 50.00000 mV/s

PreScan Definition:

-   -   Open circuit: 1.084 V (Measured)    -   Equilibration Time: 15 sec

In the CV scan, i.e., the I vs. E plot, depicted in FIG. 7; thefollowing settings were used:

Scan Definition:

-   -   Initial Potential: 0.0000 V vs open circuit    -   Vertex Potential: 0.5000 V vs open circuit    -   Vertex Potential: −0.5000 V vs open circuit    -   Final Potential: 0.0000 V vs open circuit    -   Scan rate: 50.00000 mV/s

PreScan Definition:

-   -   Open circuit: 1.263 V (Measured)    -   Equilibration Time: 15 sec

Referring to FIG. 8, an example cell, constructed as described above,was discharged at a voltage of 0.25 V at a cell operating temperature of815° C. and 760° C.

Example 2: Tungsten Cathode Cell

An example cell of the present invention was constructed according tothe procedures described in Example 1, using 10 g of tungsten oxide(WO₃) for the cathode material.

In the CV scan, i.e., the I vs. E plot, depicted in FIG. 9, the scanpotentials ranged from 0.5 V (discharge voltage) to 1.8 V. This cellgenerated peak discharge currents at 0.5 V that were >9 mA/cm².

Other Embodiments

All publications and patents referred to in this disclosure areincorporated herein by reference to the same extent as if eachindividual publication or patent application were specifically andindividually indicated to be incorporated by reference. Should themeaning of the terms in any of the patents or publications incorporatedby reference conflict with the meaning of the terms used in thisdisclosure, the meaning of the terms in this disclosure are intended tobe controlling. Furthermore, the foregoing discussion discloses anddescribes merely example embodiments of the present invention. Oneskilled in the art will readily recognize from such discussion and fromthe accompanying drawings and claims, that various changes,modifications and variations can be made therein without departing fromthe spirit and scope of the invention as defined in the followingclaims.

What is claimed is:
 1. A rechargeable electrochemical cell comprising: amolten anode; a cathode; and a non-aqueous electrolyte salt, wherein theelectrolyte salt is situated between the molten anode and the cathodeduring the operation of the electrochemical cell, the molten anodecomprises an aluminum material.
 2. The electrochemical cell of claim 1,wherein the molten anode has a density that is less than the density ofthe cathode and less than the density of the electrolyte salt; and thecathode has a density that is greater than the molten anode and greaterthan the electrolyte salt.
 3. The electrochemical cell of claim 1,wherein the molten anode comprises an aluminum material, and thealuminum material comprises elemental aluminum or an aluminum alloy. 4.The electrochemical cell of claim 1, wherein the electrochemical cellhas an operating temperature of greater than about 500° C.
 5. Theelectrochemical cell of claim 1, wherein the electrochemical cell has anoperating temperature of about 800° C. or greater.
 6. Theelectrochemical cell of claim 1, wherein the aluminum material comprisesan aluminum alloy, and the aluminum alloy comprises aluminum alloyedwith iron, silicon, gallium, bismuth, cadmium, scandium, indium, lead,mercury, thallium, tin, zinc, lithium, magnesium, antimony, copper,manganese, nickel, chromium, vanadium, titanium, zirconium, or anycombination thereof.
 7. The electrochemical cell of claim 6, wherein thealuminum alloy comprises aluminum alloyed with a eutectic amount ofiron, silicon, gallium, bismuth, cadmium, scandium, indium, lead,mercury, thallium, tin, zinc, lithium, magnesium, antimony, copper,manganese, nickel, chromium, vanadium, titanium, zirconium, or anycombination thereof.
 8. The electrochemical cell of claim 1, wherein thealuminum alloy comprises from about 0.01 to about 50 wt % of anon-aluminum alloying material.
 9. The electrochemical cell of claim 1,wherein the cathode comprises a cathode material comprising an elementalmetal, metal alloy, metal oxide, metal sulfide, or doped material. 10.The electrochemical cell of claim 9, wherein the cathode comprises acathode material that has a melting temperature that is less than orequal to the melting temperature of the anode.
 11. The electrochemicalcell of claim 9, wherein the cathode comprises a cathode material thathas a melting temperature that is greater than or equal to the meltingtemperature of the anode.
 12. The electrochemical cell of claim 9,wherein the cathode material is molten during the operation of theelectrochemical cell.
 13. The electrochemical cell of claim 9, whereinthe cathode material comprises iron, copper, chromium, vanadium,titanium, bismuth, gallium, lead, zirconium, magnesium, manganese,silicon, antimony, tin, zinc, or any combination thereof.
 14. Theelectrochemical cell of claim 9, wherein the cathode material issubstantially solid during the operation of the electrochemical cell.15. The electrochemical cell of claim 9, wherein the cathode materialcomprises one or more metal sulfides.
 16. The electrochemical cell ofclaim 9, wherein the cathode comprises manganese oxide, vanadium oxide,titanium aluminum tetrachloride, manganese chlorine aluminumtetrachloride, cobalt aluminum tetrachloride, or any combinationthereof.
 17. The electrochemical cell of claim 9, wherein the cathodematerial is capable of forming an alloy with the molten anode whencathode material is also in a molten state.
 18. The electrochemical cellof claim 1, wherein the electrolyte salt comprises: an anion selectedfrom a halide anion, a carbonate anion, a sulfate anion, a phosphateanion, an oxide anion, a sulfide anion, or any combination thereof; anda cation selected from an alkali metal cation, an alkaline earth metalcation, a rare earth metal cation, an actinide cation, a Sc cation, a Tication, a V cation, a W cation, a Mn cation, an Fe cation, a Co cation,an Al cation, an organic cation, or any combination thereof.
 19. Theelectrochemical cell of claim 1, wherein the electrolyte salt comprisesan alkali metal-aluminum halide.
 20. The electrochemical cell of claim19, wherein the electrolyte salt comprises LiAlCl₄, LiAlBr₄, NaAlBr₄,KAlCl₄, Na₃AlF₆, or any combination thereof.
 21. The electrochemicalcell of claim 1, further comprising a second electrolyte salt, whereinthe second electrolyte salt comprises an anion and a cation, and theanion is selected from a halide anion, a carbonate anion, a sulfateanion, a phosphate anion, an oxide anion, a sulfide anion, or anycombination thereof; and the cation is selected from an alkali metalcation, an alkaline earth metal cation, a rare earth metal cation, anactinide cation, a Sc cation, a Ti cation, a V cation, a W cation, a Mncation, an Fe cation, a Co cation, an Al cation, an organic cation, orany combination thereof.
 22. The electrochemical cell of claim 21,wherein the electrolyte salt comprises a binary melt material or aternary melt material.
 23. The electrochemical cell of claim 22, whereinthe electrolyte salt comprises a binary melt material comprisingaluminum halide and an alkali metal halide.
 24. The electrochemical cellof claim 23, wherein the binary melt material comprises AlCl₃ and KCl,or the binary melt comprises AlCl₃ and NaCl.
 25. The electrochemicalcell of claim 21, wherein the electrolyte salt comprises a ternary meltmaterial comprising aluminum halide, a first alkali metal halide, and asecond alkali metal halide.
 26. The electrochemical cell of claim 25,wherein the ternary melt material comprises AlCl₃, KCl, and NaCl. 27.The electrochemical cell of claim 1, wherein the electrolyte saltcomprises NaF, AlF₃, CaF₂, and BaF₂.
 28. The electrochemical cell ofclaim 27, wherein the electrolyte salt comprises from about 10 wt % toabout 20 wt % of NaF, from about 40 wt % to about 60 wt % of AlF₃, fromabout 10 to about 20 wt % of CaF₂, and from about 10 to about 20 wt % ofBaF₂.
 29. The electrochemical cell of claim 1, further comprising ahousing that contains the molten anode, the cathode, and the electrolytesalt, wherein a portion of the housing contacts the molten anode, andthe portion of the housing that contacts the molten anode comprises anelectrically conductive material.
 30. The electrochemical cell of claim29, wherein the portion of the housing that contacts the molten anodecomprises graphite.
 31. The electrochemical cell of claim 29, whereinthe housing further comprises boron carbide, aluminum boride, zirconiumdioxide, or any combination thereof.
 32. The electrochemical cell ofclaim 29, wherein the housing further comprises an air-tight chamber.33. The electrochemical cell of claim 29, wherein the housing furthercomprises a chamber that is permeable to air.
 34. The electrochemicalcell of claim 1, wherein the cathode comprises a bismuth material, andthe bismuth material comprises bismuth and tin.
 35. The electrochemicalcell of claim 1, wherein the cathode comprises a tungsten material, andthe tungsten material comprises WO₃.
 36. The electrochemical cell ofclaim 1, further comprising an anode current collector comprisingmolybdenum.
 37. The electrochemical cell of claim 1, further comprisinga cathode current collector comprising graphite.
 38. The electrochemicalcell of claim 1, further comprising a housing that comprises a chamberthat contains the molten anode, the cathode, and the electrolyte salt,and the housing comprises a heater that thermally communicates with thecathode, the molten anode, the electrolyte salt, or any combinationthereof.
 39. A rechargeable electrochemical cell comprising: a moltenanode comprising an aluminum material; a cathode comprising a bismuthmaterial, a tungsten material, or any combination thereof; and anon-aqueous electrolyte salt, wherein the electrolyte salt is situatedbetween the molten anode and the cathode during the operation of theelectrochemical cell, and the molten anode comprises an aluminummaterial.
 40. The electrochemical cell of claim 39, wherein the cathodecomprises a bismuth material, and the bismuth material comprises bismuthand tin.
 41. The electrochemical cell of claim 39, wherein the cathodecomprises a tungsten material, and the tungsten material comprises WO₃.42. The electrochemical cell of claim 39, wherein the electrolyte saltcomprises NaF, KF, CaF, LiF, KCl, NaCl, CaF₂, BaF₂, or any combinationthereof.
 43. The electrochemical cell of claim 39, further comprising ananode current collector comprising molybdenum.
 44. The electrochemicalcell of claim 39, further comprising a cathode current collectorcomprising graphite.
 45. A process for manufacturing a rechargeableelectrochemical cell comprising: heating an anode material to a moltenstate, wherein the molten anode material comprises aluminum; providing acathode material; and providing a non-aqueous electrolyte salt betweenthe anode material and the cathode material; wherein the molten anodematerial has a density that is less than the density of the cathodematerial.